1. Field of the Invention
This invention relates to the manufacture of potassium sulfate and more particularly to a method for the preparation of potassium sulfate by the reaction of potassium chloride and sulfuric acid contained in an aqueous solution under moderate reaction conditions and without isolation of intermediate potassium bisulfate.
2. Description of the Prior Art
Potassium sulfate is a well known inorganic compound which has found great utility in many areas, for example, as a fertilizer and as an intermediate for the production of useful products. Consequently, the prior art has been aware of various methods for preparation of potassium sulfate for many years. In one common procedure, potassium sulfate has been prepared by the reaction of potash (KCl) and sulfuric acid. However, it is necessary to carry out such reactions under rather severe reaction conditions in order to eliminate the hydrogen chloride evolved during the reaction and shift the reaction toward production of the potassium sulfate. An inherent problem in carrying out this process is that it is a two step reaction involving an intermediate reaction in which the potash and sulfuric acid react to form potassium bisulfate (KHSO.sub.4) with the elimination of one mole of hydrogen chloride. It has been difficult heretofore to carry the reaction to completion and eliminate two moles of hydrogen chloride in conducting this reaction in view of the thermodynamics involved in the reactions being carried out. For these reasons, it has been necessary for the art to utilize rather extreme reaction conditions, e.g. temperatures of well above 150.degree. C. and higher in order to obtain an economically attractive yield of potassium sulfate of reasonable purity from the reaction. These extreme reaction conditions however merely cause additional problems as the strong sulfuric acid and chlorine present at high temperatures cause severe corrosion problems. Therefore, a clear need remains in the art for methods by which potassium sulfate can be produced under moderate reaction conditions utilizing readily available starting materials.
In most of the prior art procedures, the key to a successful process resides in complete elimination of the HCl gas. A large body of art is available which discloses such reactions involving the production of K.sub.2 SO.sub.4 or Na.sub.2 SO.sub.4. A typical art reference of this type is U.S. Pat. No. 2,762,689 which involved the reaction of NaCl and H.sub.2 SO.sub.4 at temperatures of 200.degree. to 250.degree. C. Also in U.S. Pat. No. 2,275,825, Na.sub.2 SO.sub.4 and hydrochloric acid are prepared by the reaction of a slurry comprising water, NaCl, NaHSO.sub.4 and H.sub.2 SO.sub.4 with steam in a countercurrent reaction zone at temperatures of 120.degree. to 160.degree. C. With respect to prior art of this type, it should be noted that the aqueous sulfuric acid systems of Na.sub.2 SO.sub.4 and K.sub.2 SO.sub.4 are not generally considered to be equivalent in the recovery of desired products as they behave differently. Thus, K.sub.2 SO.sub.4 forms mixed acid salts on solution/crystallization whereas Na.sub.2 SO.sub.4 forms hydrates of either Na.sub.2 SO.sub.4 or NaHSO.sub.4 ; e.g., see "Solubilities of Inorganic and Metal Organic Compounds", Seidel, 4th Ed., Vol. 2, pps. 1126-27.
In copending application Ser. No. 545,554, filed Jan. 30, 1975, commonly assigned with this application, there is disclosed a new procedure by which potassium sulfate is produced from potash and sulfuric acid in an aqueous system under moderate reaction conditions. This prior application discloses and claims a method for the production of potassium sulfate which comprises reacting potassium chloride with potassium bisulfate in an aqueous solution at a temperature of about 65.degree.-110.degree. C., permitting the solid potassium sulfate to crystallize from the solution by cooling to a temperature to about 40.degree.-60.degree. C. and recovering the solid potassium sulfate formed. There are also described methods by which a slurry of the potassium bisulfate is prepared by reaction of KCl and sulfuric acid as well as procedures wherein the reaction is carried out in the substantial absence of hydrogen chloride evolution and wherein after removal of the solid potassium sulfate, there is provided a filtrate solution which, after excess water removal, is suitable for recycle to the plant which provides for preparation of the potassium bisulfate starting material.
In the continuous procedure of the prior application however, only a portion of the potassium bisulfate produced in the initial step can be utilized in the second step for conversion to potassium sulfate. The method of the present invention overcomes this problem by providing a process whereby potassium sulfate is produced in a single step from potash and sulfuric acid under moderate reaction conditions.